We do not know why a virus such as bacteriophage T4 codes for tRNAs that are produced during infection in apparent duplication of the host cell's supply. The participation of tRNA in protein synthesis is well documented; other less characterized roles appear to include the regulation of protein synthesis, initiation of nucleic acid replication and the participation in biosynthesis of cell walls and phosopholipids. We propose to examine the physiological role of the T4 tRNAs by characterizing the defect associated with the absence of individual tRNA species in conditionally lethal mutants of T4. Transfer RNAs are involved in highly specific interactions with various enzymes during protein synthesis. The recognition of a tRNA by its cognate aminoacyl synthetase is one of the most demanding of these interactions in terms of the specificity required to maintain the observed fidelity of protein synthesis. The second goal of this project is thus designed to provide information about those molecular features of tRNA which are recognized during the aminoacylation reaction. A genetic system has been devised which allows selection of tRNA mutants that can be aminoacylated with a new amino acid. Determination of the nucleotide sequence change and certain enzymatic properties associated with the mutant tRNAs should help reveal how the fidelity of the aminoacylation reaction is maintained.